1. Field of the Invention
The invention relates to DC balancing of ferroelectric and/or bipolar liquid crystal displays, particularly when used in reflective mode.
2. Description of the Related Art
DC balance is required of all liquid crystal displays. For twisted nematic (TN) materials, this is simply done by driving the individual cell with an AC waveform. This approach works well for TN materials because the molecules do not change physical state appreciably when the AC waveform changes electrical polarity. However, it does not work well with binary materials, such as ferroelectric liquid crystal (FLC) materials. When the polarity changes in FLC materials, the individual cell molecules change state, for instance, from on to off, turning off the cell. Thus, the individual cell must be turned off for approximately one-half of the time. This greatly reduces overall efficiency and brightness of any display built using FLCs.
One approach to solving this problem for transmissive mode FLCs is to include an additional 1/2-wave plate compensating FLC in the system. The primary imaging FLC is a 1/2-wave plate and is placed in series with the additional 1/2-wave plate, which is a single cell. The compensating FLC is switched in synchronism with the imaging FLC so that the light polarization orientation is rotated 90 degrees by the compensating FLC prior to reaching the imaging FLC. As a result, the light is in the opposite state from normal, and the polarity reversed imaging FLC will now behave as normal. When the imaging FLC is not polarity reversed, the compensating FLC is turned off, the polarization of the light is not changed, and the imaging FLC operates normally. The compensating and imaging FLCs thus both maintain DC balance, and yet the reversed state of the imaging FLC does not produce a superimposed negative period.
While this solves the problem for transmissive mode FLC operation, it does not solve the problem for reflective mode FLC operation. In reflective mode operation, the FLC is a 1/4-wave plate and light passes through the FLC to a mirror and returns back through the FLC, resulting in a total 1/2-wave retardation. However, if a 1/2-wave plate compensating FLC is used, the result of the compensating FLC is zero or full-wave retardation. Full-wave retardation produces the same result as if there were no compensating FLC at all, so that any light is still a negative. So a solution is needed to recover the efficiency in the reflective mode use of an FLC.